1. Field of the Invention
In recent years, aluminum alloy structures that have resistance against high pressures have been widely used in various applications including heat exchangers for natural gases. To meet such demands, it has been necessary to increase the strength of the exchangers against high pressure encountered. The present invention relates to a manufacturing method which helps increase the strength of joint portions by improving a solidified structure at brazed joint portions formed by brazing aluminum alloy structures.
2. Background of the Prior Art
To date, brazing materials used for brazing aluminum alloy structures are generally composed of an Al-Si type alloy when it is to be brazed with flux, or is composed of an Al-Si-Mg type alloy when it is to be brazed without flux. However, the joint formed by using such brazing materials provides a solidified structure which includes a eutectic structure, and is brittle. In particular, when a brazing material of the Al-Si-Mg type is used or when the base metal contains magnesium, the brazed joint further loses the strength.
Study has so far been conducted extensively to reinforce the brazed joints. In the case of a plate-fin type heat exchanger, for example, limitations are imposed on the size of the brazed portions of fins though it may vary depending upon the shape of protuberance of fins, amount of brazing material supplied, properties of the brazing material, and the like. In treating the plate-fin type heat exchangers, furthermore, a method has been proposed to increase the strength against the pressure relying upon the aging and hardening of a base metal of the Al-Mg-Si type by using a heat-treated aluminum alloy as a fin material, effecting a solution heat treatment thereof after the brazing, and thereafter effecting hardening and tempering. According to this method, however, the base metal is limited to a heat-treated aluminum alloy (Japanese Patent Publication No. 11948/1982).